That points back to an issue in the second mechanical fuel pump that is doing exactly the same thing the previous fuel pump was doing. I don't doubt that could be the issue, just a strange coincidence that 2 fuels would both have the same defect. I now wonder if the obstruction/issue could be in the electronic fuel pump. It is suppose to allow gas to flow freely through it regardless as to if it is turned on or not but I wonder if when it is turned off it is restricting flow.
It's not a coincidence. Modern "new" fuel pumps are all garbage made in China. I have had the same problems with mechanical pumps for 50s Cadillacs, 70s Fords, and the AD truck. Two brand new pumps for the AD failed straight from the box.
You will need to have your original pump properly rebuilt.
